Method for making solid state image sensing device

ABSTRACT

In a solid state image sensing device of p-conductivity type well, photo-electro converting region (1) are configurated to have larger area as depth increases, so that excessive electric charges generated in the p-conductivity type well are easily transferred from expanded peripheral parts (7) at the bottom (1b) to channel (3), without being undesirably transferred downward through thin p-conductivity type region 6 to substrate (4), and smear electric charges which has been generated in a thin p-conductivity type well under the photo-electro converting region in the conventional device is suppressed, to effectively decrease the smear phenomenon.

FIELD OF THE INVENTION AND RELATED ART STATEMENT

1. Field of the Invention

The present invention relates generally to a solid state image sensingdevice and method for making the same and particularly to a solid stateimage sensing device of p-conductivity type well structure.

2. Description of the Prior Art

In recent years, large improvements in the characteristics of solidstate image sensing devices has been made, especially devices of interline transfer type CCD show superior characteristics than conventionalimage pick up tubes. Even in such improved solid state image sensingdevice however, so called smear phenomenon, which is caused by mixing ofa part of electric the charges generated by incident light into thetransfer means, has not been eliminated. Smear phenomenon is, therefore,the important problem to be solved. In order to improve the smearphenomenon, a p-conductivity type well structure is consideredeffective.

A solid state image sensing device having the p-conductivity type wellstructure is, for instance, disclosed in 1983 lEEE InternationalSolid-State circuits Conference Friday Feb. 25, 1983. SESSION XVIII,FAM18.7 p264-265. However, the conventional solid state image sensingdevice, even though having the p-conductivity type well structure, hassuch shortcomings which do not sufficiently eliminate the smearphenomenon. Also, the sensitivity for long wavelength light is lowbecause photo-electro converting regions thereof are shaped undesirably.Furthermore, because the area of the thin p-conductivity type welldesign which is underneath the photo-electro region is small,elimination of blooming phenomenon is insufficient.

OBJECT AND SUMMARY OF THE INVENTION

One object of the present invention is to provide a solid state imagesensing device wherein smear phenomenon is drastically decreased.

A second object of the present invention is to provide a solid stateimage sensing device wherein blooming phenomenon is sufficientlysuppressed.

A third object of the present invention is to provide a solid stateimage sensing device which has high sensitivity.

Still another object of the present invention is to provide a solidstate image sensing device which is highly integrated.

These and other objects are accomplished by a solid state image sensingdevice comprising the following structure.

A semiconductor layer is formed on a semiconductor substrate.

At least one photo-electro converting region is formed in thesemiconductor layer.

At least one gate electrode is formed on the semiconductive layer.

At least one vertical transfer means is disposed away from thephoto-electro converting region. Electric charges generated from thephoto-electro converting region upon irradiation of light thereto aretransferred to the vertical transfer means when electric potential isapplied on the gate electrode.

The photo-electro converting region has a shape in which the size of thetop portion thereof is smaller than that of the bottom portion.

With respect to the method for making the solid state image device inaccordance with the present invention, a first object of the inventionis to provide a method for making a solid state image sensing devicewherein smear phenomenon is drastically decreased.

A second object of the present invention is to provide a method formaking solid state image sensing device wherein blooming phenomenon isalleviated.

A third object of the present invention is to provide a method formaking a solid state image sensing device which has high sensitivity.

Still another object of the present invention is to provide a method formaking a solid state image sensing device is highly integrated.

These and other objects are accomplished by a method for making solidstate image sensing device comprising the steps.

A first layer of a first conductivity type is formed on a semiconductorsubstrate of a second conductivity type.

A second layer of a said second conductivity type is then formed on saidfirst layer.

An impurity of said first conductivity type from the surface of saidsecond layer, is then selectively introduced thereby forming a thirdlayer of said first conductivity type. This results in at least onephoto-electro converting region surrounded by first and third layers ofsaid first conductivity type.

In a specific embodiment, the first semiconductor layer or the secondsemiconductor layer is formed by the introduction of impurities.

In another specific embodiment, the first semiconductor layer or thesecond semiconductor layer is formed using an epitaxial growth method.

This invention has various advantages among which are the following.

The invention can provide a solid state image sensing device whereinsmear phenomenon is drastically decreased, and also a method for makingthe same.

The invention can provide a solid state image sensing device whereinblooming phenomenon is alleviated.

The invention can provide a solid state image sensing device which hashigh sensitivity.

The invention can provide a solid state image sensing device which ishighly integrated.

While the novel features of the invention are set forth withparticularity in the appended claims, the invention both as toorganization and content will be better understood and appreciated,along with other objects and features thereof, from the followingdetailed description of the preferred embodiment taken in conjuctionwith the drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross sectional view showing a preferred embodiment of asolid state image sensing device in accordance with the presentinvention,

FIG. 2(a), FIG. 2(b), FIG. 2(c) and FIG. 2(d) are cross-sectional viewsshowing progression of manufacturing steps of the solid state imagesensing device embodying method for making in accordance with thepresent invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The solid state image sensing device and method for making the same aredescribed hereunder in connection with a few preferred embodiments andby reference to the accompanying drawings.

FIG. 1 shows a cross-sectional view of a preferred embodiment of thesolid state image sensing device. Each unit of the device has on asemiconductor substrate 4, for instance, of silicon, a p-conductivitytype well 5 and therein a photo-electro converting region 1 whichreceives light incident thereto and issues electrons corresponding tothe amount of input light. The p-conductivity type well 5 also hasvertical transfer means, for example, a vertical CCD channel region 3wherethrough the electrons accumulated in the photo-electro convertingregion 1 are transferred outside the circuit by application of a controlsignal on gate electrode 2 formed on the vertical CCD channel 3. Areverse bias voltage is impressed across the n-conductivity typesubstrate 4 and the p-conductivity type well layer which comprises thickpart 5 and thin parts 6, thereby is suppressed blooming phenomenon. Thegate electrode 2 is formed on an insulation film, for instance, thinSiO₂ film 9, in a manner to be insulated from the vertical CCD channelregion 3.

As shown in FIG. 1, the photo-electro converting region 1 is configuredto have larger area in lower part than smaller area in upper part. Thatis, peripheral parts 7 of the photo-electro converting region 1 areformed being bent toward the vertical CCD channel 3. Therefore, thedistance between the photo-electro converting region 1 and the verticalCCD channel 3 is narrower than the corresponding parts in theconventional solid state image sensing device with a p-conductivity typewell, and a depression layer which is on the interface between thep-conductivity type well 5 and the photo-electro converting region 1extend towards the vertical CCD channel 3.

As a result of the above-mentioned configuration, electric chargesgenerated in the p-conductivity type well region 5 due to obliqueincident light on the photo-electro converting region 1 is reduced.Furthermore, smear electric charges generated in the p-conductivity typewell 5 by the oblique incident light through the photo-electroconverting region 1 decrease and are instead easily captured by thephoto-electro converting region 1. Accordingly, electric chargesundesirably captured by the vertical CCD channel 3 are reduced.Therefore smear phenomenon is drastically reduced.

The configuration of the photo-electro converting region 1 has a widerarea in the lower part and the widest area is this lower part.Therefore, the thin p-conductivity type area 6 formed between thesubstrate part 4 and the photo-electro converting part 1 has a widearea. Therefore, excessive electric charges generated in thephoto-electro converting region 1 are effectively evacuated by thepunch-through effect between the thin part of the p-conductivity typewell 6 and the n-conductivity type substrate 4 and the reversely biasedphoto-electro converting region 1. Therefore, blooming phenomenon ,previously generated by electric charges travelling towards the n-region3, is effectively suppressed.

FIG. 2(a), FIG. 2(b), FIG. 2(c) and FIG. 2d) show a progression ofmanufacturing steps for one embodiment of the manufacturing process inaccordance with the present invention. First, as shown in FIG. 2(a), onan n-conductivity type silicon substrate 4, by means of thermaldiffusion followed by a heat treatment or by means of ion-implantationfollowed by a heat treatment, a p-conductivity type impurity doped layer6 is formed. As the impurity, for instance, B is usable. Next, in asimilar way an n-conductivity type impurity doped layer 1 is formed onthe impurity doped layer 6, as shown in FIG 2(b). Thereafter, as shownin FIG. 2(c), a p-conductivity type impurity, for instance, B, having around boundary face (i.e., diffused front) is selectively diffused fromthe surface of the n-conductivity type impurity doped layer 1 to such adepth to reach the underlying p-conductivity type impurity doped layer6, thereby to form p-conductivity type impurity diffused regions 5.

In the above-mentioned ion-implantation, by appropriately selecting ionsof appropriate diffusion coefficients and appropriate temperatures ofheat treatment, ion-implanting materials, for making the p-conductivitytype impurity doped layer 6, the n-conductivity type impurity dopedlayer 1 and the p-conductivity type purity doped layer 5, and bysubsequently making only one heat treatment, selected diffused regions 5are formed in a smaller number of steps.

For instance, by appropriate amount of p impurity ions, for instance,1×10¹² cm⁻² of B ions and an appropriate amount of n-conductivity typeimpurity ions, for instance, 0.8×10¹² cm⁻² of As, and subsequentlycarrying out one heat treatment on these implanted impurities, becauseof the difference of diffusion coefficients between As ions and B ions,the configuration of FIG. 2(b) is obtainable, wherein the B-diffusedlayer 6 becomes deeper than As-diffused layer 1'.

Thereafter, known methods can be used to form p⁺ -conductivity typechannel stopper regions 11 by ion-implantation of B, isolation regions 8made of oxide film and vertical CCD channels 3 of n-conductivity typemade by diffusing As. Thin oxide insulation film 9 covering thep-conductivity type diffused regions 5 and the oxide insulation film 8as well as transfer electrodes 2 made of polycrystalline silicon areformed using known steps, the result being shown in FIG. 2(d). Heattreatments necessary to form these regions can be the same type of heattreatments used to form the p-conductivity type of region 6, then-conductivity type region 1 and the p-conductivity type region 3. Ifthe p-conductivity type impurity layer 6 and n-conductivity typeimpurity doped layer 1 are formed by known epitaxial growth method thethicknesses of these layers can be controlled and the results obtainedare satisfactory.

According to one manufacturing method embodying the present invention,since the photo-electro converting regions 1, are formed to have a shapeof larger area at the bottom 1b than surface part 1a, smear phenomenonobserved in prior solid state image sensing devices is prominentlydecreased.

In the solid state image sensing device of the p-conductivity type wellconstruction, such as the present invention, it is important to evacuatethe excessive electric charge from the photo-electro converting region 1through the thin p-conductivity type purity doped layer 6 under thephoto-electro converting region 1 by utilizing layer 6 as a channel tothe n-conductivity type substrate 4 to suppress the so called bloomingphenomenon.

However, since the thin p-conductivity type impurity doped layer 6becomes the channel for the evacuation of the electric charges, theability to suppress the blooming becomes proportional to the area of thethin p-conductivity type doped layer 6.

Since the lateral diffusion front of the p-conductivity type puritydoped region 5 is accurately controllable, the area of the thinp-conductivity type impurity doped region 6 can be controlled accuratelyand there is no fear of unduly narrowing the area. Thus, suppression ofblooming is assured. The suppressing of the blooming phenomenon is alsoassured even if the image sensing device is highly integrated.

Furthermore, according to the manufacturing method of the presentinvention, since the thin p-conductivity type impurity doped region 6 isdisposed in a deep position from the surface of the wafer, the devicehas a high sensitivity even for a long wavelength light, and hence, asolid state image sensing device having a high sensitivity wide range ofwavelength is manufacturable.

Furthermore, apart from the above-mentioned embodiment, wherein theperipheral boundary of the n-conductivity type region expandsmonotonously with the depth, the manner of the expanding of thisperipheral boundary need not necessarily be monotonous, but some littlewaving of the peripheral boundary is admissible as long as the boundaryface is generally a surface of a substrate.

Apart from the above-mentioned embodiment where CCD type solid stateimage sensing device is described, the present invention is alsoapplicable to MOS type solid state image sensing devices. Furthermore,the invention is applicable for solid state image sensing devices of onedimensional and two dimensional types.

While specific embodiment of the invention have been illustrated anddescribed herein, it is realized that modification and changes willoccur to those skilled in the art. It is therefore to be understood thatthe appended claims are intended to cover all modifications and changesas fall within the true spirit and scope of the invention.

We claim:
 1. Method for making a solid state image sensing devicecomprising the steps of:forming a first layer of a first conductivitytype on a semiconductor substrate of a second conductivity type which isopposite to said first conductivity type; forming a second layer of saidsecond conductivity type on said first layer; selectively introducing animpurity of said first conductivity type from a surface of said secondlayer to reach said first layer where it is underlying said secondlayer, for forming at least two impurity diffused regions of said firstconductivity type, resulting in at least one photoelectric convertingregion which is surrounded by said first layer and two of said impuritydiffused regions of said first conductivity type, said selectivityintroducing step forming said photoelectric converting region of a shapeto have a larger horizontal section at a lower portion than at an upperportion to reduce smearing phenomena; forming at least one chargetransfer region on said impurity diffused regions; and forming at leastone gate electrode above said impurity diffused regions and said chargetransfer region.
 2. Method for making solid state imaging sensing devicein accordance with claim 1, wherein said substrate is of ann-conductivity type, said first layer is of a p-conductivity type andsaid photo-electro converting region is of n-conductivity type.